War and Conquest: Battle Rules for Birthright is a reference work about conducting large scale combat at the Birthright domain level for use with the Birthright Rulebook.
From the publisher blurb
The conflict of peoples is the crucible of history. War and conquest are core themes of the Birthright setting. Despite the fact that battles figure so prominently in the setting and genre, the setting has long lacked a sensible system to conduct warfare in a way that fits with the existing game mechanics.
War and Conquest contains rules to portray Birthright military forces as leveled units using methods that mesh much more smoothly with the standard system of portraying domain level organizations. Troop types, racial characteristics and terrain effects are retained by the system, preserving all aspects of the original warcard system, but fitting them into a system that parallels standard Birthright game mechanics. Conquest of provinces is incorporated into the system, as are other important aspects of conflict in Birthright like sieges, mercenaries, levies and naval combat.